1. Field of the Invention
The present invention relates generally to the art of substrates for electrode structures and more specifically to substrates useful in zinc-bromine batteries. Still more specifically, the invention relates to the preparation of such substrates using novel fabrication techniques, including glass mat reinforcement or a slurry process similar to that employed in the paper industry.
2. Description of the Prior Art
Zinc-bromine batteries have been designed for use in electric vehicles, for bulk energy storage and for other battery applications which can take full advantage of the high energy density and low manufacturing costs offered by the zinc-bromine system. Because the production cost of battery parts is critical, ease in manufacturing is highly desirable. Extensive experimentation has focused on developing a high performance, low cost bipolar electrode system for use in zinc-bromine batteries.
Zinc-bromine batteries are described in Zito U.S. Pat. No 4,482,614 issued Nov. 13, 1984 for "Zinc-Bromine Battery With Long Term Stability". In this patent, a number of electrode substrate materials are described including those made from thermoplastic polymers such as polyvinylidene fluoride, acrylonitrile-butadiene-styrene or polyvinylchloride. The basic structure of zinc-bromine batteries is also shown in such patent and in U.S. Pat. No. 4,169,816 issued Oct. 2, 1979 to Tsien for "Electrically Conductive Polyolefin Compositions". Other conductive electrode compositions are shown in U.S. Pat. No. 4,294,893 issued Oct. 13, 1981 to Iemmi, et al. for "Graphite-Resin Composite Electrode Structure, And A Process For Its Manufacture" and U.S. Pat. No. 4,124,747 issued Nov. 7, 1978 to Murer, et al. for "Conductive Polyolefin Sheet Element". Further examples of metal halide battery structures include those described in U.S. Pat. No. 3,642,538 issued Feb. 15, 1972 to Zito for "Metal Halide Battery" and U.S. Pat. No. 4,105,829 issued Aug. 8, 1978 to Venero for "Metal Halogen Batteries And Method of Operating Same".
By following a typical discharge-charge cycle, the operation of zinc-bromine batteries can be easily understood. During charging, zinc is plated at the negative electrode and bromine is evolved from the positive electrode. Bromine reacts with a soluble complexing agents provided in the system to form a second liquid phase. The bromine-rich phase is then removed from the stack of electrodes and separated by gravity in a catholytic storage region. An emulsion of the bromine-rich phase and the aqueous bromine phase is fed back to the stack. The electrical reactions involved during charge of the zinc-bromine battery system can be shown as follows: EQU Zn.sup.++ +2e.fwdarw.Zn.degree. (anode) EQU 2 Br.fwdarw.Br.sub.2 (aq)+2e (cathode) EQU Zn.degree.+Br.sub.2 .fwdarw.ZnBr (discharge)
During discharge the reverse reactions occur, forming the original zinc-bromide solution and liberating the energy absorbed during charging The separator prevents direct mixing of anolyte and catholyte.
One of the key components in any zinc-bromine battery is the bipolar electrode substrate. The optimal substrate for use in zinc-bromine systems should have the following combination of properties: low resistivity, ease of manufacture at low cost, flexibility, strength, chemical stability and minimal expansion during use.
A common mode of failure for batteries using the prior art has been expansion of the electrode structure which causes warpage and leads to eventual system failure. As the brominated electrolyte interfaces with the electrode, bromine absorbs onto the surface of carbon fillers used in prior systems, and the polymer matrix relaxes. Due to this absorption and relaxation, the overall size of the electrode increases. In most systems, the electrode is fixed in place at the periphery, and the increase in size can only be accommodated by warpage. Thus, it is essential that a bipolar electrode substrate exhibit the desired physical properties listed previously, yet not be subject to warpage.
Recent attempts at making zinc-bromine batteries have focused on electrode systems which are flat and thin (about 0 015 to 0.120 inches) with the size and shape depending on the application and the specific design requirements. The electrode should provide a resistivity of less than 5 .OMEGA.-cm, adequate strength, flexibility and chemical stability for the life of the battery.
Prior conductive plastic composites have included compositions consisting of ethylene/propylene copolymer, Cabosil.RTM. (or fumed silica), glass fiber, pitch fiber and carbon-black. The purpose of the glass and pitch fiber was to increase flexural strength, and the Cabosil.RTM. was provided to facilitate better mixability during compounding operations. Carbon-black was loaded to approximately 58 v/o based on the polymer. In some systems, non-conductive top, bottom and side borders have been used for mounting and manifolding of the electrodes. Such electrodes and insulating plastic frames were prepared using a co-extrusion process described in U.S. Pat. No. 4,521,359 issued to Hsue C. Tsien on June 4, 1985 and entitled "Method of Coextruding Plastics to Form a Composite Sheet".
Polymer systems using carbon-black as a filler have received special attention because they combine the inherent properties of polymers (such as toughness, flexibility and chemical resistance) with a relatively high conductivity. They are also light in weight and can be mass produced using inexpensive processes such as extrusion and injection molding.
Chemical stability has been the most difficult problem to overcome with such systems. Tensile strength declined significantly upon exposure to bromine, and the average molecular weight of the polymer systems also decreased as the length of exposure increased. It was believed, based on these tests, that bromine was being absorbed onto the substrate over time, causing unacceptable expansion and warpage, along with deterioration of the electrodes' physical properties. See FIG. 1 of the drawings. Bromination, unlike chlorination, is extremely selective to the chemistry of the polymer matrix used, and the tertiary hydrogens of polypropylene systems react approximately twenty thousand (20,000) times faster with bromine than the secondary hydrogens in polyethylene.
Improved substrates for electrode systems, such as those involved in the bipolar electrodes of zinc-bromine batteries, would represent a significant advance in the art.